In the building industry, there is a continuous need to decrease the energy costs related to heating and cooling indoor rooms. One of the key factors that improve the energy management is insulation layers, which come in different physical forms, such as stone wool, polymeric foams or different fibers. The insulation layer inhibits the heat transfer from inside to outside or, alternatively, from outside to inside and thus either keeps the interior warm in cold weather conditions or, alternatively, cold in hot weather conditions.
In recent years, new kinds of building materials have been used for managing the thermal efficiency of buildings. These new materials are based on phase change materials (PCMs). A PCM is latent thermal storage material using chemical bonds to store and release heat, which is capable of absorbing and releasing high amounts of latent heat during melting and crystallization, respectively.
During a hot day, for example, a solid PCM may gradually melt, thereby absorbing the thermal energy that would otherwise have had heated up the building. During the ensuing colder night, the previously molten PCM re-solidifies (or crystallizes), thereby releasing the thermal energy absorbed during the day and so heating the building up at night.
Phase change materials can be organic or inorganic substances, and paraffins and vegetable oils and/or fats are frequently used as organic PCMs because of their low cost and toxicity.
PCM can be introduced in matrices made of different materials or applied to a coating. See, e.g., U.S. Pat. Nos. 4,003,426, 4,528,328, 5,053,446, US20060124892 (WO2006/062610), WO98/04644, and WO2004/044345.
In order to conveniently use the PCMs in building applications, the PCMs have been so far incorporated into matrix polymers that absorb and retain the phase change materials, even at temperatures above the melting point of the PCMs, thus making it possible to manufacture the resulting PCMs composite materials into slabs, panels or other shapes that are easily mounted in a wall. However, most matrix polymers suffer from multiple drawbacks such as limited PCM absorption capacity, substantial loss of PCM by exudation during lifetime, and an increase in brittleness of the matrix polymer upon PCM absorption. A solution to the above problems has been proposed in WO2006062610A2.
Nowadays, there is a strong trend towards renewable materials that have a low environmental impact, and the building sector is no exception. It therefore does not come as a surprise that manufacturers of building material have developed different PCM composite materials where the fossil fuel-based materials are replaced by renewable and/or biological materials.
U.S. Pat. No. 6,574,971 discloses a method for sourcing PCMs from vegetal oil or animal fats instead of from fossil fuel.
However, these PCMs can suffer from several drawbacks when incorporated in polymeric matrices, such as, for example, their tendency to supercool.
Supercooling, also known as undercooling, occurs when a liquid is cooled below its melting point without becoming solid.
In PCMs, whose thermal effect is almost entirely based on the energy absorbed or released during the phase change between liquid and solid, supercooling is a very undesirable property.
An unwanted situation arises when, for example, after having absorbed heat during a first day and having changed from solid to liquid, the PCM cools down to a temperature that is below its melting point during the night without solidifying. On the second day, the supercooled PCM may already be in the liquid state because of supercooling and may thus be unable to absorb appreciable quantities of heat by PCM from solid to liquid. This may lead to poor thermal performance during the second day.
In the context of the invention, the temperature at which the PCM solidifies upon supercooling and which is below the melting temperature is called the crystallization temperature (TC).
Some PCMs suffer more than others from supercooling. For example, pure paraffin suffers less from supercooling as described in U.S. Pat. No. 6,765,031 than PCMs based on vegetal oils and/or animal fats. Because paraffin is based on non-renewable fossil fuels, it is less preferred in compositions that need to source the phase change materials from renewable sources.
There is therefore a need to provide a composite material that comprises a maximum of renewably sourced materials, in particular renewably sourced PCMs, but does not suffer from insufficient PCM absorption and where the tendency of PCMs to supercool is at least partially or entirely prevented.